Bushing assemblies are generally located at a connecting joint between a base member and a motion inducing member in such applications as machinery, airplanes, boats and vehicular transportation. The function served by bushing assemblies is to accommodate motion between the motion inducing member and the base member and to dampen out oscillatory or vibratory motions. Such vibratory motions may include a wide range of frequencies, ranging from small to large amplitudes, and may occur in various directions of the bushing assembly including radial, axial as well as rotary vibrations. In most applications it is desirous to protect the base member from such vibratory motions.
The arrangement of the base member and the motion inducing member with respect to the connecting joint are dependent on the particular design and application. In most applications, the base member has an arm intermediate of two ends, one end having a mating member forming a part of the connecting joint and the other end affixed to a body wherein it is desirous that vibratory motions be eliminated. Such a body may be a frame of an automobile, sensitive equipment or a drive shaft when the bushing assembly is used as a coupling.
The motion inducing member generally has an elongated arm intermediate of two ends, one end affixed to a motion inducing source and the other end having a mating member forming a complimentary part of the connecting joint. The motion inducing source generates various motions including vibratory motions that are accommodated along the elongated arm to the bushing assembly in the connecting joint. As mentioned above, such accommodated motions relative to the bushing assembly may be radial, axial or rotary.
The bushing assembly is affixed within the housing of the connecting joint to control static and dynamic motions as well as accommodate movement between the motion inducing member and the body. In particular a primary purpose of a bushing assembly is to substantially reduce or dampen the vibratory motions at the connecting joint. Damping occurs when the energy of the vibratory motion is dissipated by the bushing assembly. Damping controls the response of the base member at or near the resonance frequency of the motion inducing member. In particular, damping diminishes the amplitude of the vibration at the resonance frequency. Increasing amounts of damping diminishes the amplitude of the vibratory motions. At or near a resonance frequency, damping is the only means of controlling motion because the other two factors affecting response, mass and stiffness transfer energy. Uncontrolled resonant response can provide excessive base member motion which, in turn, leads to impaired reliability or structural failure.
The most common type of bushing assembly is a rubber bushing. Rubber bushings generally comprise annular elongate inner and outer members with elastomer disposed therebetween. Such bushings are used to control and transmit movement at the connecting joint, but have limited capability in damping vibratory motions. Damping provided by rubber bushings is a function of the hysteresis property of the elastomer. In general, rubber bushings can be said to provide little damping.
One form of bushing assembly which can provide improved damping are fluid filled bushings. Fluid filled bushings generally include a cylindrical elongate inner rigid member, an elongate outer rigid sleeve member concentrically disposed and radially spaced from the inner member and a resilient means disposed beteen the inner member and outer sleeve member wherein the resilient elastomeric means defines a pair of circumferentially spaced and diametrically opposed fluid filled chambers fluidly connected by an elongate restricted passageway. In response to vibratory motions along the radial direction of the bushing assembly between the inner member and the outer sleeve member fluid is displaced from one chamber via the restricted passageway to the second chamber in a direction opposite to the vibratory motion. In particular, when a first chamber is contracted by a vibratory motion in a radial direction, the fluid is displaced therefrom through the restricted passageway to an expanding second chamber in opposite radial direction. In the reverse cycle of the vibratory motion, when the first chamber is expanding and the second chamber is contracting, the fluid is reversibly moved through the restricted passageway. As can be seen, an oscillatory motion of the fluid is generated within the restricted passageway between two chambers. The oscillatory fluid in the restricted passageway creates a mass or inertia resistance to the pumping forces of the chambers which act together in a mass-spring relationship resulting in damping of the vibratory motions along the radial direction.
An example of such a fluid filled bushing is disclosed in U.S. Pat. No. 3,642,268. The bushing therein disclosed utilizes hydraulic fluid displaceable between two diametric chambers via a restricted orifice. The chambers are located in the bushing along a first radial direction whereas along a second radial direction perpendicular to the first radial direction is a solid rubber member which extends along the axial direction of the bushing. Such a fluid filled bushing exhibits low stiffness and high damping along the first radial direction dependent on the flow characteristics between the chambers and the fluid properties as described heretofore and high stiffness and low damping along the axial and rotary directions.
A fluid filled bushing for damping vibrations in both the radial and axial directions is disclosed in U.S. Pat. No. 4,667,942. The bushing therein disclosed utilizes hydraulic fluid displaceable between two sets of two chambers, the first set of two chambers provides damping in the axial direction and the second set of two chambers provides damping in the radial direction. The two sets of two chambers are fluidly interconnected via two restricted passageways. Vibratory motions in the radial direction are dampened by the transfer of fluid between the first set of two chambers via the two restricted passageways and vibratory motions in the axial direction are dampened by the transfer of fluid between the second set of two chambers via the two restricted passageways.
While the fluid filled bushings disclosed in the referenced patents function satisfactorily for their intended use of damping vibratory motions in the radial and/or axial directions, there is a need for a fluid filled bushing assembly which can satisfactorily dampen rotary vibratory motions. In certain applications, the bushing assembly in the connecting joint is exposed to rotary vibratory or also described as rotary oscillatory motion. In one application a bushing assembly would supplant a coupling providing damping between the drive member and the driven member. In another application a bushing assembly could be a focal point between two arms wherein the bushing assembly would provide damping of the oscillatory motion. The bushing assemblies of the prior art are inadequate to provide damping of rotary oscillatory motions in such applications.